Thermal hysteresis has been a significant contributing factor to frequency instability of RF oscillators and in particular, quartz crystal oscillators (XO) which are currently the most widely used frequency reference technology for high-performance consumer applications, such as mobile phones and GPS receivers, due to the corresponding high performance-to-cost ratio.
Current technology used to achieve maximum frequency stability in XO uses interpolation of a lookup table of frequency and temperature measurements to minimise temperature induced frequency error. However, this approach cannot remove the effects of hysteresis and is typically limited to about ±0.25 ppm stability over a temperature range of −30° C. to +80° C.
U.S. Pat. Nos. 7,259,637 and 7,466,209 disclose systems which attempt to model thermal hysteresis in XO by offering two possible frequency versus temperature curves for each of the two directions of temperature. U.S. Pat. No. 7,259,637 discloses a system in which a separate lookup-table of temperature versus frequency values is used for when the temperature is increasing to when the temperature decreasing. U.S. Pat. No. 7,466,209 discloses a system which aims to account for temperature versus frequency directional dependence by providing a single function that models a frequency curve for when the temperature is increasing and another frequency curve for when the temperature is decreasing.
As consumer expectations become increasingly demanding, the frequency stability requirements become progressively more stringent. For example, as GPS technology has become more and more widespread—so has the consumer expectation of using a GPS receiver indoors. This typically requires operating the receiver in weak signal conditions which can be in the order of −165 dBm, pushing the limits of the current state of the art.
One method of improving receiver performance is increasing the integration periods in the code correlation. However, this has the consequence that the tolerance of the receiver to frequency reference instabilities is dramatically reduced through a square law relationship. Thus, if the contribution of hysteresis to these instabilities can be mitigated then it will allow GPS receivers to take advantage of longer integration times, therefore offering improved sensitivity without significantly increasing core processing power.